In a typical optical inspection apparatus used for performing precision measurements, the optical system is typically suspended above the object being measured and is adjustable in the Z-axis direction (vertical) while the object is supported on an X, Y axis mechanical stage, where precise control of the X, Y, Z axes facilitates the measurement process. An example of such a system is disclosed in U.S. Pat. No. 5,668,665.
There are a number of geometric error components associated with a measurement machine's total volumetric error. For example, there are three translations and three angular errors (pitch, roll and yaw) for each axis and three squareness errors, one for each axis. The errors associated with the Z-axis (vertical axis) also have a compounding effect on the other axes. The Z-axis pitch error manifests itself as a translation error in the Y-axis measurement. The Z-axis yaw error manifests itself as a translation error in the X-axis measurement. The Z-axis roll error compounds the problem further by manifesting itself into both a X-axis and a Y-axis translation error. Adding the zoom lens into the error equation, the higher the magnification, the higher the translation error for the same angular error.
There is, therefore, a need for a Z-axis mount for a lens assembly in an optical inspection system that reduces the errors typically associated with the Z-axis.